Out-of-registration colors (image displacement) is an obstinate problem to a multi-color laser printer equipped with a plurality of laser scanning units. In order to obtain an accurate multi-color print onto a receiving medium by overcoming this problem, displacements in a main scan direction must be eliminated first of all. This elimination can be realized by matching scan-start-timings of the plural scanning units in the main scan direction. Respective rotary polygons provided to each scanning unit reflect the laser beam from the source to a photo-conductive drum. At this time, clocks driving motors that drive the polygons are started in phase. Assume that the number of rotations of the polygons agree with each other. However, the phases toward reflecting faces of respective polygons do not always agree with, and also the phases on the photo-conductive drums do not always agree with because of mounting errors of polygons as well as errors in rotating-detection-signals of the motors.
While the laser beam scans in main scan direction, the photo-conductive drums rotate, thus if the scanning starts with some time-lag, the drums have rotated by the angle corresponding to the time-lag, which causes deviation in sub-scan direction. In other words, time-lag in recording onto the drum in the main scan direction is reflected in the out-of-registry in the sub-scan direction. Even if the time-lag in the main scan direction is small, out-of-registry in the sub-scan direction must be corrected with the precision as high as not more than one pixel. A technique of correcting the out-of-registry in the sub-scan direction is disclosed in the Japanese Patent Application Non-examined Publication No. H07-160084. This technique teaches that sync circuits between respective scanning units are prepared responsive to phase control status of each motor driving the respective rotary polygons. Then timing signals for recording are generated with the precision as high as not more than one pixel.
However, this registry adjusting method according to the publication No. H07-160084 does not clarify a method of how to agree the timing signals of respective scanning units at the initial stage. The timing signals (=beam detect signal) are used for agreeing the recording timings of video signals in the main scan direction. The video signals can be obtained by receiving the laser beam from the respective scanning units. Out-of-registry of an image in the sub-scan direction can be adjusted with the precision not more than one pixel. However, it can be adjusted in a unit of, e.g. a half pixel or one quarter pixel, namely in a unit of one pixel divided by an integer, thus the image registry can be adjusted discontinuously.
In the registry adjusting device of a laser beam printer discussed above, it is required that the recording timing be adjusted continuously first of all, thereby controlling with accuracy the spin of motors that drive the rotary polygons.